curves

I am having difficulty understanding your question. I am guessing your are talking about where the scan stop for each curve. The condition for stopping curve tracing for each curve is roughly 1.8 times of the maximum power dissipation specified. It is just a coincidence that the curves that are multiple of -20Vs meet the stop criteria at a higher current point compares to curves for 0, -10, -30, etc.

There is nothing wrong with your plot

You can do that if you want. But for many of us we are only interested in curves within the max plate dissipation.